b. a sampling scheme—a combination of sampling
plans with switching rules and possibly a provision for discontinuance of inspection. In this standard
the terms “sampling scheme” and “scheme per- formance” will be used in the restricted sense
described in Sec. 11.1.
c. a sampling system—a collection of sampling
schemes. This standard is a sampling system indexed by lot-size ranges, inspection levels, and AQLs.
3. PERCENT NONCONFORMING AND
NONCONFORMITIES PER HUNDRED UNITS 3.1
EXPRESSION OF NONCONFORMANCE. The
extent of nonconformance of product shall be expressed either in terms of percent nonconforming or in terms of non-
conformities per hundred units.
3.2 PERCENT NONCONFORMING. The percent non-
conforming of any given quantity of units of product is one hundred times the number of nonconforming units divided
by the total number of units of product, i.e.:
3.3 NONCONFORMITIES PER HUNDRED UNITS.
The number of nonconformities per hundred units of any given quantity of units of product is one hundred times the
number of nonconformities contained therein one or more nonconformities being possible in any unit of product
divided by the total number of units of product, i.e.:
It is assumed that nonconformities occur randomly and with statistical independence within and between units.
4. ACCEPTANCE QUALITY LIMIT AQL
4.1 USE. The AQL together with the Sample Size Code
Letter, is used for indexing the sampling plans provided herein.
4.2 DEFINITION. The AQL is the quality level that is the
worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.
Note: The use of the abbreviation AQL to mean Acceptable Quality Level is no longer recommended.
4.3 NOTE ON THE MEANING OF AQL. The concept
of AQL only applies when an acceptance sampling scheme with rules for switching between normal, tightened and
reduced inspection and discontinuance of sampling inspec- tion is used. These rules are designed to encourage suppliers
to have process averages consistently better than the AQL. If suppliers fail to do so, there is a high probability of being
switched from normal inspection to tightened inspection where lot acceptance becomes more difficult. Once on tight-
ened inspection, unless corrective action is taken to improve product quality, it is very likely that the rule requiring dis-
continuance of sampling inspection will be invoked.
Although individual lots with quality as bad as the AQL can be accepted with fairly high probability, the designation of an
AQL does not suggest that this is necessarily a desirable quality level. The AQL is a parameter of the sampling
scheme and should not be confused with a process average which describes the operating level of a manufacturing
process. It is expected that the product quality level will be less than the AQL to avoid excessive non-accepted lots.
The sampling plans in this standard are so arranged that the probability of lot acceptance at the designated AQL depends
upon sample size, being generally higher for large samples than for small samples for a given AQL. To determine the
specific protection to the consumer at a given AQL, it is nec- essary to refer to the operating characteristic curves which
are provided in this standard of the corresponding scheme and its constituent plans.
The AQL alone does not describe the protection to the con- sumer for individual lots or batches, but more directly relates
to what is expected from a series of lots or batches provided the provisions of this standard are satisfied.
4.4 LIMITATION. The designation of an AQL shall not
